Induction motor with a circumferentially slitted squirrel cage rotor

ABSTRACT

A rotor for an induction motor is provide. The rotor includes a core built with stacks of a plurality of steel sheets and includes a plurality of rotor slots that are radially arranged. The rotor further includes a plurality of conductor bars contained in the plurality of rotor slots, respectively, and end-rings attached to both longitudinal ends of each of the plurality of conductor bars. The rotor further includes at least one slit formed inward from an outer periphery of the rotor along a perimeter of the rotor, wherein the slit has a depth deep enough to form a groove portion in at least some region of each of the plurality of conductor bars.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to and the benefit of Korean PatentApplication No. 10-2021-0169466 filed on Nov. 30, 2021, the disclosureof which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to an induction motor with acircumferentially slitted rotor.

BACKGROUND

The description in this section merely provides background informationrelated to the present disclosure and does not necessarily constitutethe prior art.

Whereas industrial induction motors generally rotate at a constantspeed, induction motors for vehicles run at variable speeds, i.e., startand stop operations occur frequently and repeatedly. Since inductionmotors for vehicles are run on a battery, they require high efficiencyto cover travel distances. To increase efficiency, the speed ofinduction motors for vehicles is becoming increasingly higher.Incidentally, the induction motors for vehicles are exposed for longperiods of time to vibration produced on startup and external excitationfrom driving the vehicle. In line with the trend toward higher speeds,these factors need to be considered in terms of design to ensure thedurability of the rotor.

In induction motors, the mechanical rotation rate of the rotor and therotation rate of the stator’s rotating field do not match, unlike insynchronous motors. Also, the torque varies depending on the differencein rotation speed between the stator’s rotating field and the rotor.

FIG. 1 shows slip-torque characteristics of a typical induction motor ofthe related art. FIG. 2 shows slip-current characteristics of a typicalinduction motor of the related art.

The ratio between the rotation rate of the stator’s magnetic field andthe rotation rate of the rotor is called a “slip,” and the slip-torquecharacteristics of a typical induction motor are as shown in FIG. 1 .Torque at startup (i.e., high slip state) is lower than torque in a lowslip region which is a main operation area. On the other hand, asexemplified in FIG. 2 , generally, the current induced at startup islarger than the current induced during operation, which results in lowefficiency.

FIG. 3 shows how typical slip-torque characteristics of an inductionmotor of the related art change with increasing rotor resistance. FIG. 4shows how typical slip-current characteristics of an induction motor ofthe related art change with increasing rotor resistance.

As exemplified in FIG. 3 , the slip at which maximum torque is obtainedincreases in proportion to the resistance of the rotor (secondary side),and the maximum torque itself is constant, which is referred to as aproportional shifting effect in the field of induction motors. Where thesecondary side has a larger resistance, more starting torque isproduced, and a smaller current is induced at startup.

When an induction motor has a squirrel-cage rotor, the rotor may includedeep bars or double squirrel-cage conductor bars, such as thoseexemplified in FIGS. 5A and 5B, in order to reduce starting current andincrease starting torque using this proportional shifting effect.

FIGS. 5A and 5B exemplify a deep bar and a double squirrel-cageconductor bar as a conductor bar included in a rotor of the related art.

The deep bar has the shape of a conductor bar that runs longitudinallyfrom the outer periphery of the rotor to the inner periphery. Thus,leakage reactance increases toward the inner periphery. Since the slipat startup is large, the frequency at the secondary side increases, andcurrents are concentrated on the outer periphery of the rotor whereleakage reactance is small, i.e., in conductive portions on the rotor’ssurface. Accordingly, the cross-sectional area of the conductors throughwhich current flows is virtually reduced, which is similar in effect toan increase in the resistance of the rotor. This conceptually explainshow the starting torque of the induction motor using deep bars isincreased.

A squirrel-cage induction motor has a double cage construction in whichconductor bars are divided into outer conductors arranged on the outerperiphery and inner conductors arranged on the inner periphery. Theouter conductors are conductors having higher specific resistance thanthe inner conductors. The outer conductors and the inner conductors maybe connected by a bridge-like member. When the slip at startup is largeand the frequency at the secondary side is very high, the ratio ofleakage reactance in the impedance at the secondary side is much higherthan the ratio of resistance. Thus, at startup, the current at thesecondary side is limited by leakage reactance. Therefore, currents areconcentrated on the outer conductors where specific resistance is large,leaving little current flowing through the inner conductors.Accordingly, the starting torque may be increased. In the case of adouble squirrel-cage, since the specific resistance of the innerconductors is smaller than the specific resistance of the outerconductors, most of the current flows through the inner conductors whilethe rotor is operating at a rated revolutions per minute (rpm), i.e.,when the slip is small and the frequency at the rotor is low, therebyachieving higher efficiency.

The double squirrel-cage may be formed in an empty space where no outerconductors are arranged on the outer periphery - i.e., nothing ischarged - for the purpose of reducing eddy currents and alsosignificantly reducing losses caused by pulse width modulation (PWM) inthe stator.

Where conductor bars are formed by die casting in holes formed by rotorslots included in stacks of a plurality of steel sheets, constructing adouble squirrel cage with different kinds of materials may makemanufacturing difficult and increase costs. Also, a rotor with acomplicated structure similar to the double squirrel cage may raise aconcern about a decrease in durability, due to the increased centrifugalforce in high-speed induction motors. Accordingly, it is desirable thatthe rotor of an induction motor designed to run at high speed has asimple structure.

When there are deep bars, a magnetic flux created in the conductors onthe inner periphery follows a long path to pass through air gaps, whichmay cause losses because of a large amount of magnetic flux leakageflowing toward the slots.

SUMMARY

The present disclosure provides a rotor structure that can increase thestarting torque of an induction motor and additionally can improve thecooling effect.

According to at least one embodiment, the present disclosure provides arotor for an induction motor. The rotor includes a core built withstacks of a plurality of steel sheets and includes a plurality of rotorslots that are radially arranged. The rotor also includes a plurality ofconductor bars contained in the plurality of rotor slots, respectively,and end-rings attached to both longitudinal ends of each of theplurality of conductor bars. The rotor further includes at least oneslit formed inward from an outer periphery of the rotor along aperimeter of the rotor, wherein the slit has a depth deep enough to forma groove portion in at least some region of each of the plurality ofconductor bars.

An induction motor rotor, according to the present disclosure, has theeffect of improving the starting torque of the induction motor. Also,the induction motor’s characteristics can be easily adjusted by theshape of slits to be made in a post-machining process. Further, as thesurface area of the outer periphery of the rotor increases, the coolingcharacteristics of the rotor can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows slip-torque characteristics of a typical induction motor ofthe related art.

FIG. 2 shows slip-current characteristics of a typical induction motorof the related art.

FIG. 3 shows how, in the related art, typical slip-torquecharacteristics of an induction motor change with increasing rotorresistance.

FIG. 4 shows how, in the related art, typical slip-currentcharacteristics of an induction motor change with increasing rotorresistance.

FIGS. 5A and 5B exemplify a deep bar and a double squirrel-cageconductor bar as a conductor bar included in a rotor of the related art.

FIGS. 6A and 6B illustrate a rotor comprising a slit on the outerperiphery according to an embodiment of the present disclosure.

FIGS. 7A and 7B are a cross-sectional views of slits formed in a rotoraccording to an embodiment of the present disclosure.

FIG. 8 illustrates slits arranged parallel to one another according toan embodiment of the present disclosure.

FIG. 9 illustrates helical slits according to an embodiment of thepresent disclosure.

FIGS. 10A and 10B illustrate side cross-sectional views of a slitstructure according to an embodiment of the present disclosure.

FIG. 11 shows slip-torque characteristics of an induction motoraccording to an embodiment of the present disclosure.

FIG. 12 shows an improvement in torque characteristics of an inductionmotor according to an embodiment of the present disclosure.

FIG. 13 shows an improvement in the efficiency of an induction motoraccording to an embodiment of the present disclosure.

FIGS. 14A and 14B show a structure of one end of a helical slit forimproving cooling characteristics according to a further embodiment ofthe present disclosure.

DETAILED DESCRIPTION

Some embodiments of the present disclosure are described below withreference to the accompanying drawings. In the following description,like reference numerals preferably designate like elements, although theelements are shown in different drawings. Further, in the followingdescription of some embodiments, a detailed description of knownfunctions and configurations incorporated herein is omitted for thepurpose of clarity and for brevity.

Additionally, alphanumeric codes such as first, second, i), ii), a), b),and the like, in numbering components are used solely for the purpose ofdifferentiating one component from the other but not to imply or suggestthe substances, the order, or sequence of the components. Throughoutthis specification, when parts “include” or “comprise” a component, theyare meant to further include other components, not excluding thereofunless there is a particular description contrary thereto.

FIGS. 6A and 6B illustrate a rotor comprising a slit on the outerperiphery according to an embodiment of the present disclosure. FIG. 6Ais a partial perspective view illustrating part of a rotor 10 accordingto an embodiment. FIG. 6B illustrates a conductor bar 100 according toan embodiment of the present disclosure.

Referring to FIGS. 6A and 6B, a rotor 10 according to an embodimentincludes a core 110, a plurality of conductor bars 100, and a pair ofend-rings 120.

The core 110 is built with stacks of a plurality of steel sheetscomprising a plurality of rotor slots that are radially arranged. Theconductor bars 100 are contained in the rotor slots, respectively. Theend-rings 120 are attached to both longitudinal ends of each of theconductor bars 100. The rotor 10 of FIG. 6A shows only one end-ring 120of the pair of end-rings 120, since FIG. 6A is only a partial view ofthe rotor 10.

The rotor 10, according to an embodiment, further includes at least oneslit 130 made into the outer periphery of the rotor 10 around theperimeter of the rotor 10. The slit 130 has a depth deep enough to forma groove portion in at least some region (i.e., a portion) of each ofthe plurality of conductor bars 100.

The conductor bar 100 is illustrated in FIG. 6B. The conductor bar 100is shown separately only to describe the slit 130 or the grooved portionformed on the outer periphery of the conductor bar 100. However, theconductor bars 100 each are not preliminarily formed in this manner. Theslit 130 (or groove portion) of the conductor bar 100, according to anembodiment, is formed by machining the outer periphery of the rotor 10in a post-machining process after the preassembling of the rotor 10.

The outer periphery of the conductor bar 100 including the slit 130,according to an embodiment, has a larger resistance due to the slit 130,thereby increasing the starting torque and reducing the startingcurrents.

FIGS. 7A and 7B are a cross-sectional views of slits formed in a rotoraccording to an embodiment of the present disclosure. FIG. 7A is across-sectional view of a preassembled rotor 10 taken along the centerof symmetry of the conductor bar 100. FIG. 7B is a cross-sectional viewof a rotor 10 with slits formed in a post-machining process, taken alongthe center of symmetry of the conductor bar 100.

A rotor 10, according to an embodiment, is first manufactured by awell-known traditional method, such as the molding of conductor bars bydie casting, for example. Next, slits 130 are formed on the outerperiphery of the manufactured rotor 10 using a machining method such ascutting machining and wire electrical discharge machining (WEDM), forexample. By machining, the slits 130 are formed by removing part of thecore 110 and conductor bars 100 of the rotor 10. A plurality of slits130 are arranged in an axial direction of the rotor 10. This lengthensthe path in which currents flow along the outer peripheries of theconductor bars 100 because it follows the shape of the slits 130, thusincreasing resistance.

The shape, arrangement, and size of the slits 130 may be selectedaccording to desired motor characteristics.

FIG. 8 illustrates slits 130 arranged parallel to one another accordingto an embodiment of the present disclosure.

For the convenience of the description, FIG. 8 schematically illustratesthe configuration and positions of slits 130 that simplify the rotor 10,where the slits 130 are arranged parallel to one another. Although theillustrated embodiment shows by example that a plurality of slits 130are arranged at regular intervals, the arrangement of slits 130 is notlimited to this, and the intervals between the slits 130 and the widthof the slits 130 may vary. For example, the intervals between the slits130 near both ends of the rotor 10 may be narrower or wider than theintervals between the slits 130 in a central area of the rotor 10.Similarly, helical slits 132 to be described below may have a fixedpitch, and, if necessary, the helical slits 132 may have differentpitches in some regions.

FIG. 9 illustrates helical slits 132 according to an embodiment of thepresent disclosure.

As illustrated in FIG. 9 , the helical slits 132, according to anembodiment, may be helically formed on the outer periphery of the rotor10 around the axis of rotation of the rotor 10.

As can be readily predicted by those having ordinary skill in the art,the helical slits 132 formed along the outer periphery of the rotor 10may provide a rotational vibration reduction effect, as is normally thecase with slits skewed on the outer periphery of the rotor 10 at anangle to the axis of rotation.

The helical slits 132 may be made through opposite sides of the rotor10, or the helical slits 132 may be formed in such a way that both endsthereof are positioned between opposite sides of the rotor 10. In thelatter case, the helical slits 132 may be tapered in depth at both endsand seamlessly connect to unmachined parts of the outer periphery of therotor 10.

FIGS. 10A and 10B illustrate a side cross-sectional view of a slitstructure according to an embodiment of the present disclosure.

FIGS. 10A and 10B illustrate a slit structure used in simulation (3DFEA) for examining characteristic changes in the induction motor causedby the slits 130 arranged parallel to one another according to anembodiment. FIG. 10A represents a part where the slits 130 are notformed (referred to as a base for convenience, meaning a region wherethe slits 130 are not formed). FIG. 10B represents a part where theslits 130 according to an embodiment are formed. For example, the ratioof the width of a unit base to the width of a unit slit with respect tothe axis of the rotor 10 may be 8:2. An analysis was carried out under avarying slip condition, assuming that the phase current supplied to thestator (not shown) is 3 Arms. In the analysis, copper losses in thestator and the rotor 10 were considered.

FIG. 11 shows slip-torque characteristics of an induction motoraccording to an embodiment of the present disclosure.

Referring to FIG. 11 , an induction motor comprising a squirrel-cagerotor with circumferential slits 130, according to an embodiment of thepresent disclosure, exhibited an increase in starting torque. Eventhough the maximum value of torque and the slip at which maximum torqueis produced decrease slightly, the high starting torque of the inductionmotors for vehicles may be advantageous in terms of performance andefficiency since they start and stop frequently during operationcompared to induction motors for general purposes which run at aconstant speed.

FIG. 12 shows an improvement in torque characteristics of an inductionmotor according to an embodiment of the present disclosure. FIG. 13shows an improvement in the efficiency of an induction motor accordingto an embodiment of the present disclosure.

Referring to FIG. 12 , a slit model exhibited an increase of about 8.7 %in torque at startup compared to a base model. Referring to FIG. 13 ,the slit model exhibited an increase of 2.8 % in torque at startup.

In the induction motor, according to an embodiment, the slits 130 formedon the outer periphery of the conductor bars 100 increase the surfacearea of the conductor bars 100 along the length of the conduction bars100. Therefore, the slits 130 lengthen the path in which currentsinduced by a magnetic flux in a high slip state flow, thus virtuallyleading to an increase in resistance and, consequently, an increase instarting torque. The depth of the slits 130 may be selected byconsidering the amount of slip required to increase starting torque andthe depth of the path of a main magnetic flux for that slip.

The rotor 10, according to an embodiment, has a significantly increasedsurface area on the outer periphery by comprising slits 130 on the outerperiphery. Induction motors, which have the issue of large amounts ofheat generation in the rotor 10 compared to synchronous motors, mayachieve an improvement in cooling characteristics by the increase insurface area caused by the slits 130. Accordingly, the induction motor,according to an embodiment, may provide improvements in overall torqueperformance as well as in starting torque. Moreover, since the slits 130according to an embodiment may be made in a post-machining process, thestarting characteristics may be improved without redesigning theconductor bars 100. In addition, in the case of a rotor with noidentical slits, the starting characteristics may be variouslyimplemented by easily adjusting the width, depth, and intervals of theslits 130.

FIGS. 14A and 14B show a structure of one end of a helical slit 132 forimproving cooling characteristics according to a further embodiment ofthe present disclosure.

The helical slit 132, according to an embodiment, may provide anadditional improvement in the cooling of the rotor 10 since the helicalslit 132 has a slit opening 140 on opposite sides of the rotor 10.Refrigerant from the outside may enter through the slit opening 140formed on one side of the rotor 10.

The slit opening 140 includes: a first edge 142 where the direction ofhelix of the helical slit 132 forms an acute angle with a side of therotor 10; a second edge 144 where the direction of helix of the helicalslit 132 forms an obtuse angle with the side of the rotor 10; and athird edge 146 where a bottom surface of the helical slit 132 and theside of the rotor 10 meet.

Refrigerant supplied to one side of the rotor 10 from which the firstedge 142 is skewed in the direction of rotation of the rotor 10 may besmoothly supplied into the helical slit 132. To facilitate the entry ofrefrigerant into the helical slit 132, the rotor 10 may further includea protrusion 150 protruding from a side portion 152 of the rotor 10contiguous to the first edge 142. In other words, the end-rings 120 mayinclude a protrusion 150 in some region.

Alternatively, a threaded hole (not shown) may be formed in the sideportion 152 of the end-ring 120 where the protrusion 150 is to beincluded, and a separate member corresponding to the protrusion 150 maybe fastened into the threaded hole. For example, the separate member maybe a bolt (not shown), and a head of the bolt may serve as theprotrusion.

The protrusion 150 may be formed in such a way that, as the rotor 10rotates, external air including refrigerant is introduced into thehelical slit 132. The overall torque performance of induction motors maybe enhanced by improving the cooling efficiency of the rotor 10.

Although one embodiment discloses conductor bars 100 arranged in a rowalong the circumference of the rotor 10, the present disclosure is notlimited to this, and slits may be formed on the outer periphery of adouble squirrel-cage rotor.

Although embodiments of the present disclosure have been described forillustrative purposes, those having ordinary skill in the art shouldappreciate that various modifications, additions, and substitutions arepossible, without departing from the idea and scope of the claimeddisclosure. Therefore, embodiments of the present disclosure have beendescribed for the sake of brevity and clarity. The scope of thetechnical idea of the present embodiments is not limited by theillustrations. Accordingly, one of ordinary skill in the art wouldunderstand that the scope of the claimed disclosure is not to be limitedby the above explicitly described embodiments but by the claims andequivalents thereof.

What is claimed is:
 1. A rotor for an induction motor, the rotorcomprising: a core built with stacks of a plurality of steel sheets andcomprising a plurality of rotor slots that are radially arranged; aplurality of conductor bars contained in the plurality of rotor slots,respectively; and end-rings attached to both longitudinal ends of eachof the plurality of conductor bars, wherein the rotor further comprisesat least one slit formed inward from an outer periphery of the rotoralong a perimeter of the rotor, wherein the slit has a depth deep enoughto form a groove portion in at least some region of each of theplurality of conductor bars.
 2. The rotor of claim 1, wherein the slitis a circular slit disposed in a plane perpendicular to an axis ofrotation of the rotor, and a plurality of circular slits are disposed ina lengthwise direction of the rotor.
 3. The rotor of claim 1, whereinthe slit is a helical slit which is formed along a helical path aroundan axis of rotation of the rotor.
 4. The rotor of claim 3, wherein thehelical slit is formed on the outer periphery of the core.
 5. The rotorof claim 4, wherein the helical slit is formed so that both ends thereofare positioned between opposite sides of the rotor, and depth and/orcross-sectional area of the helical slit is gradually reduced.
 6. Therotor of claim 4, wherein the helical slit comprises a slit openingformed through opposite sides of the rotor past the end-rings.
 7. Therotor of claim 6, wherein the slit opening comprises: a first edge wherea direction of helix of the helical slit forms an acute angle with aside of the rotor; a second edge where the direction of helix of thehelical slit forms an obtuse angle with the side of the rotor; and athird edge where a bottom surface of the helical slit and the side ofthe rotor meet, wherein the slit opening further comprises a protrusionprotruding from a side portion of the rotor contiguous to the firstedge.
 8. The rotor of claim 7, wherein the side portion of the rotorcontiguous to the first edge comprises a threaded hole, and theprotrusion is fastened into the threaded hole.
 9. The rotor of claim 8,wherein the protrusion is a head of a bolt screwed into the threadedhole.